Dosage effects of polyphagous genes, PhB, PhS and Sek, of the silkworm, Bombyx mori

Two mutants, Ph and Sek, are known to be polyphagous in the silkworm, Bombyx mori. The feeding capacity of those mutants was compared by feeding newly hatched larvae on a semi synthetic diet that does not contain atractive component, Morin (Ohnuma, 1999). Since in these strains each majour gene is known to be accompanied by a number of modifiers for its expression, comparisons were made between different phenotypes segregated in a cross made between the two strains. They were fed on the synthetic diet for 48 hours after hatching and feeding capacity of each segregated phenotypes was estimated by the ratio of grown larvae to the total number tested. Each polyphagous major gene was marked with a linked morphological gene, i.e., Ph**B with Ze, Ph**s, with + Ze and Sek with pe or + pe. F1 females obtained from a cross between Ph and Sek were crossed with homozygous males either for Ph or Sek (Table 1). As shown in Fig.1 five types of cross combination, A, B, C. D and E were produced and feeding capacity was recorded separately for each segregated phenotypes in each plot. The results were put together in a table (Table 7), in which grown larvae rates are shown for different genotypic segregants both in total sum and in an average of each experimental plot. In a case of heterozygote, Ph was fur higher in their grown larvae rate than Sek, but when comparisons were made in homozygote the difference disappeared between the two (No.4 and No.9). Similarly, in case of homozygote for one gene and heterozygote for another gene no difference was seen, each giving higher grown larvae rate than 99%. In conclusion, the results obtained were in the order of, 4>=3>2 (homozygote)>2 (heterozygote)>1. More simply, the feeding capacity was proportional to the number of polyphagous genes.